1. Field of the Invention
The present invention relates to electron-beam writing technology, and particularly to a technique for high-precision, high-speed writing.
2. Description of the Prior Art
In recent years, attempts have been made to perform writing using an electron-beam writing device (for example, see Japanese patent laid-open No. 273583/1996 (Hei 8-273583)). A predetermined pattern is written by such an electron-beam writing device in which a deflector deflects an electron beam so that the electron beam will be scanned over the surface of a target substrate material. The deflector typically consists of two units, i.e. a high-precision deflection unit and a high-speed deflection unit. The high-precision deflection unit specifies the position of a target area (hereinafter called a writing field), and the high-speed deflection unit deflects the electron beam within the writing field so that the electron beam will be scanned to create a writing pattern.
For example, the following describes a conventional method of writing an oblique line on a substrate material with reference to FIGS. 1 and 2 wherein FIG. 1 shows the principles of the conventional method by which the oblique line is written on the substrate and FIG. 2 shows how to write a line segment of the pattern shown in FIG. 1.
The electron beam writing device typically includes a high-precision D/A converter for driving the high-precision deflection unit and a high-speed D/A converter for driving the high-speed deflection unit. The high-precision D/A converter for driving the high-precision deflection unit operates at 16 or 18 bits, and the high-speed D/A converter for driving the high-speed deflection unit operates at fewer bits than the high-precision D/A converter, for example at 12 bits.
Therefore, when writing the same line, the deflector driven by the high-precision D/A converter can obtain the line with higher positional accuracy than that driven by the high-speed D/A converter. On the other hand, the deflector driven by the high-speed D/A converter can obtain the line at higher speed than that driven by the high-precision D/A converter.
In FIG. 1, a writing field 801 is a writing area controlled by the high-precision deflection unit, and a writing field 802 is a writing area controlled by the high-speed deflection unit. In this case, the high-precision D/A converter drives the high-precision deflection unit to specify the starting position of the writing pattern. Then the high-speed D/A converter drives the high-speed deflection unit to deflect an electron beam so that the electron beam will be scanned within the writing field 802 to create the writing pattern.
For example, as shown in FIG. 1, it is assumed that a line 803 consisting of line segments 803a, 803b, and 803c connected at their ends is written. In this case, a point 804a is the starting position of writing the line segment 803a, and a point 804b is the end position of the line segment 803a. The point 804b is also the starting position of the line segment 803b, and a point 804c is the end position of the line segment 803b. The point 804c is also the starting position of the line segment 803c, and a point 804d is the end position of the line segment 803c. In other words, the end position of the line segment 803a corresponds to the starting position of the line segment 803b, and the end position of the line segment 803b corresponds to the starting position of the line segment 803c. 
When writing such a line 803, at the beginning, the high-precision D/A converter drives the high-precision deflection unit so as to deflect an electron beam to be positioned at point 804a, and after that, the high-speed D/A converter drives the high-speed deflection unit and hence the deflector to deflect an electron beam so that the electron beam will be scanned in such a manner to write the line segment 803a first, and then continue writing the line segments 803b and 803c in this order. In the following, it is described, with reference to FIG. 2, how to write the line segments 803a, 803b, and 803c. 
FIG. 2 shows how to write a line segment, for example, the line segment 803a. In this case, the length of the line segment 803a is L, and a unit distance (distance corresponding to one dot) corresponding to the resolving power of the high-speed D/A converter is LSO. The line segment 803a is separated into X and Y components to determine the position of the endpoint 804b of the line segment 803a. Then an electron beam is irradiated and scanned to write the line segment 803a. In the prior art, the unit distance corresponding to the resolving power of the high-speed D/A converter is set to the same value as that corresponding to the resolving power of the high-precision D/A converter.
Then, using the length L of the line segment 803a and the unit distance LS0 corresponding to the resolving power of the high-speed D/A converter, the number of scan clocks (Count) required for the high-speed D/A converter to write from the starting point 804a to the endpoint 804b is calculated. The Count is defined by the following equation (1):Count=Round (L/LS0)where “Round” denotes to round off L/LS0. For example, in FIG. 2, if the value of L/LS0 for the line segment 803a is smaller than “4.5” the Count is “4” and if the value of L/LS0 is equal to or larger than “4.5” the Count is “5”. In this prior-art description, it is assumed that the value of L/LS0 is equal to or lager than “4.5”. Therefore, the number of scan clocks (Count) required to write from the point 804a to the point 804b is “5”.
Then the length L of the line segment 803a is separated into X and Y components to convert the lengths of the X and Y components in an equation using the number of scan clocks (Count). Specifically, these conversions are made using the above-mentioned number of scan clocks (Count) and the unit distance LS0 of the high-speed D/A converter according to the following equations (2):X=(LSO×Count)×cos θ, andY=(LSO×Count)×sin θ.Thus the end position of the line segment 803a is determined using the unit distance LS0 of the high-speed D/A converter and the number of scan clocks (Count) required to write from the starting point to the endpoint.
Then the electron beam is irradiated dot by dot (at every interval LSO). Specifically, as shown in FIG. 2, the electron beam is irradiated at the starting position 804a (805a), and then at points 805b, 805c, 805d, 805e, and 805f in this order at regular intervals, Lso. Since the point 805f is the fifth dot (Count=5) from the point 805a, the electron beam is irradiated up to the point 805f. 
During this operation, the electron beam irradiated on the substrate scatters inside the substrate to cause an effect as if portions in the neighboring area are irradiated by the electron beam. Therefore, although the electron beam is irradiated at regular intervals of the unit distance, the scattering events of the electron beam inside the substrate results in writing the line segment 803a. 
The line segments 803b and 803c are written in the same manner to create the line 803.
If the value of L/LSO is smaller than “4.5” since the Count is “4” the electron beam is irradiated up to the point 805e. 
In the prior art, however, the unit distance LSO corresponding to the resolving power of the high-speed D/A converter needs to be shortened when higher positional accuracy is necessary to write the starting point and endpoint of a line. To this end, more bits are required, causing a problem that the writing speed is inevitably decreased. On the other hand, the unit distance LSO corresponding to the resolving power of the high-speed D/A converter needs to be lengthened for high-speed writing of a line. To this end, the number of bits has to be reduced, causing a problem that high positional accuracy cannot be obtained.
Further, when the endpoint of a line segment is calculated in the above-mentioned manner, there is a further problem that causes an endpoint error ΔL806. Although the point 804b as the end position of the line segment 803a exists between the points 805e and 805f, the electron beam cannot be irradiated at any point between the points 805e and 805f merely by using the unit distance corresponding to the resolving power of the high-speed D/A converter. In other words, the electron beam is irradiated either up to the point 805e or the point 805f. This is why the endpoint error 806 is inevitably caused.